This invention relates to imager systems, and in particular imager systems utilizing high fill-factor image sensor arrays.
Recent developments in the field of image sensing technology have focused on the switch from relatively low fill-factor image sensor arrays, which utilize an array of isolated sensors to detect light, to relatively high fill-factor image sensor arrays that utilize a continuous layer of sensor material formed over an array of pixel circuits. Each pixel circuit of these high fill-factor image sensor arrays includes an access transistor and a contact (i.e., a metal pad) that is connected to the lower surface of the sensor material layer. A continuous transparent bias layer (e.g., indium tin oxide (ITO)) is typically formed on an upper surface of the continuous sensor material layer. Each sensor operates on the principal of integrating a charge representative of the quantities of radiation incident on the sensor. When an image is to be captured by the image sensor array, radiation (e.g., light or X-rays) conveying the image strikes the sensor material layer, which responds by freeing electrons and holes that generate a local current in the sensor material layer between the pixel contacts and the continuous bias layer. These local currents change the potentials on the underlying pixel contacts according to the amount of light incident thereon. The potential on each pixel contact is periodically xe2x80x9creadxe2x80x9d by sequentially turning on the access transistors to couple the pixel contacts to a series of charge-sensing amplifiers. The differences between the various potentials read from the pixel contacts are then used to reconstruct the captured image.
One well-known type of high fill-factor image sensor array utilizes hydrogenated amorphous silicon (a-Si H) sensor material for real time imaging (see R. A. Street et al., xe2x80x9cLarge Area Image Sensor Arraysxe2x80x9d, in Technology and Applications of Amorphous Silicon, Editor R. A. Street, Springer Series in Materials Science 37, Springer-Verlag, Berlin, 2000, chapter 4, p 147, for a general description of the structure of the arrays). Such a-Si H sensor arrays are particularly advantageous for X-ray imaging because they present a relatively large size image sensor array. In the direct detection approach, incident high-energy radiation (e.g., X-ray photons) is directly converted to a charge by the sensor. In the indirect detection approach, a phosphor converter absorbs high-energy radiation (e.g., X-ray photons) and generates a proportional amount of visible light that is then converted to a charge by the sensor.
An obvious problem associated with the use of continuous sensor material layers is crosstalk between adjacent pixels, which occurs when the continuous sensor layer allows conduction between pixel contacts. This form of crosstalk directly reduces the resolution of the image sensor array because a sharp feature will be blurred into neighboring pixels. As mentioned above, as the sensor material located over one pixel is illuminated, the charge from the illumination builds up on that pixel""s contact. This shifts the voltage on that contact towards the bias voltage level applied to the continuous bias layer. If the sensor layer allows lateral conduction, then the potential difference between adjacent pixels will result in conduction from one pixel to the next. Experimentally, in image sensor arrays utilizing continuous a-Si H sensor material layers, this form of crosstalk has been observed with varying magnitude, but primarily is a problem as the pixel reaches saturation (i.e., approaches forward bias). See Rahn J. T. et al. xe2x80x9cHigh-Resolution High Fill Factor a-Si H Sensor Arrays for Medical Imaging,xe2x80x9d Proc. of SPIE, Vol. 3659, pp. 510-517, 1999.
Another problem associated with high fill-factor image sensor arrays, which is also a problem with all pixilated structures, is the rejection of high spatial frequency signals. Because the pixilation of an image sensor array acts as a sampling function, high spatial frequency signals are aliased into lower frequencies. High fill-factor image sensor arrays (described above) reduce the impact of aliasing, but do not eliminate it. In many imaging systems, the image source can be designed to reject high spatial frequencies, for example, by designing the focus of the optical system to blur the image and reject high spatial frequencies. In addition, indirect x-ray detection typically does not have much of a problem with aliasing, since the phosphor screen rejects high spatial frequencies. However, in direct detection imagers that do not include optical blurring, the effects of aliasing can be clearly seen. Even if the imager can be designed so that high spatial frequencies are filtered on the imager, the noise will also be aliased and the total noise power increased, which reduces the Detector Quantum Efficiency (DQE) of the imager.
Accordingly, what is needed is a high fill-factor image sensor array that significantly reduces crosstalk between adjacent pixels. What is also needed is a high fill-factor image sensor array that filters high spatial frequency signals prior to imaging.
The present invention is directed to a high fill-factor image sensor arrays in which the image resolution is improved by reducing crosstalk between adjacent pixels. This crosstalk reduction is achieved by the various embodiments of the present invention by clamping the sensor voltage (e.g., the voltage across the photodiode of each pixel) to prevent saturation, and/or by maintaining the pixel contact at a fixed voltage.
In accordance with a first embodiment, a high fill-factor imager system includes a scanning control circuit for generating gate voltage signals on a plurality of gate lines, a bias voltage source, and an imager including a plurality of pixels arranged in an array. Each pixel of the array includes a sensor (e.g., a photodiode) for generating a charge, a storage capacitor for storing the charge, and an access transistor connected between the storage capacitor and an associated data line of the array. The sensor includes a first terminal (e.g., an anode) that is maintained at a predetermined bias voltage by the bias voltage source, and a second terminal (e.g., a cathode) connected to a first terminal of the storage capacitor. A second terminal of the storage capacitor is connected to a system voltage source. At the beginning of an imaging cycle, the second terminal (cathode) of the sensor is reset such that a predetermined voltage exists across the sensor. Light (or other radiation) striking the sensor generates a proportional charge therein. This charge is stored by the storage capacitor, and is passed to the associated data line during a subsequent readout operation.
Pixel clamping in the first embodiment is achieved either by maintaining the bias voltage well below the gate off voltage of the access transistors, or by periodically pulsing the gate voltage to drain excess charge during exposure and between readout cycles. Note that the description of this invention assumes n-type transistors and a sensor biased negative with respect to the data line. This invention is not limited, however, to this polarity. According to the first approach, the scanning control circuit generates either a gate on voltage, which turns on the access transistors of a column of pixels during readout/reset, or a gate off voltage that turns off the access transistors. By maintaining the bias voltage at least one threshold voltage of the access transistors below the gate off voltage, excess charge is drained from the storage capacitor onto the data line through the turned-off access transistor when the cathode voltage gets too close to the bias voltage, thereby preventing the photodiode (sensor) from reaching saturation. A potential problem with this approach is that draining charge onto the data line during the readout cycle of another pixel connected to that data line can result in unwanted crosstalk. Therefore, according to the second approach, in addition to the gate on and gate off voltages, the scanning control circuit generates additional voltage pulses during exposure at times between the readout operations of the pixels connected to the data line. These additional voltage pulses, which have a voltage level that is less than that of that used for readout operations, allow charge from one pixel to drain onto a data line without disrupting readout operations from other pixels connected to the same data line.
In accordance with a second disclosed embodiment, a high fill-factor image sensor array includes circuitry similar to that of the first embodiment, but each pixel also includes a clamp transistor connected in parallel with the storage capacitor between the system voltage source and the sensor. The clamp transistor is controlled by a global clamp voltage that is at least one threshold voltage above (or below, if polarities are reversed) the bias voltage, thereby causing the clamp transistor to drain excess charge from the storage capacitor before the photodiode saturates.
In accordance with a third disclosed embodiment, a high fill-factor image sensor array includes circuitry similar to that of the first embodiment, but each pixel also includes a cascode transistor connected in series between the storage capacitor and the sensor. The cascode transistor is controlled by a global control voltage that is at least one threshold voltage of the cascode transistor above the bias voltage, thereby causing the cascode transistor to maintain the second terminal of the sensor (e.g., the cathode of the photodiode), which is connected to the pixel contact, at a fixed voltage level. When the sensors of the pixel array are formed using a continuous film of a-Si H, maintaining all of the pixel contacts at the fixed voltage level prevents crosstalk by minimizing potential differences between adjacent pixel contacts.
In accordance with a fourth embodiment, both the clamp transistor of the second embodiment and the cascode transistor of the third embodiment are combined to enhance crosstalk reduction.
In accordance with a variation of the third and fourth disclosed embodiments, a resistive film is provided between the sensors (e.g., photodiodes) of the various pixels that acts as a filter for high spatial frequencies. Typically, the continuous a-Si:H sensor layer includes relatively heavily doped (n+) regions formed over each pixel contact that are separated from adjacent pixels by undoped (intrinsic) a-Si:H. In contrast, the resistive film is formed by a continuous, relatively lightly doped (n) layer that connects all of the relatively heavily doped regions. This resistive film allows localized areas of high illumination to diffuse into adjacent pixels before imaging (readout), thereby filtering high spatial frequencies and avoiding image aliasing.